1. Field of the Invention
The present invention generally relates to power generation systems and more particularly to steam-augmented gas turbine engines in which the compressor turbine mass flow is nearly constant such that the compressor-turbine performance (isentropic efficiency) remains on design from a Cheng point to a stoichiometric point.
2. Description of the Related Art
In addition to the usual problems and restrictions associated with land-installed power plants such as fuel efficiency, cost, pollution control, and power, shipboard engines have special (i.e., stringent) restrictions as to space and weight. Further, the parameters of fuel efficiency and power are even more important in naval power plants, where conflicting requirements such as long patrolling range and high speed may each be achieved, as required.
To meet the above goals, various engines that have been developed for land-based power plants have been applied to shipboard power facilities. One system is a gas turbine based on a simple-cycle Brayton engine, designated the LM2500 and built by General Electric. Other engines have also been utilized which have serial compressors for increasing performance parameters such as power, fuel efficiency, etc.
Another type of system utilizes steam-augmented gas turbine (SAGT) engines which are extremely efficient and ecologically-benign. Commercial versions of this type of engine utilize steam/water flow rates up to 16% by weight of the airflow rate.
Steam-injected systems generally exhibit higher efficiency and increased specific horsepower than non-steam-injected systems. Such systems include many well-known gas turbines produced, for example, by Allison and General Electric. SAGT engines have been used for land-based applications in power-producing utilities and in manufacturing plants that require process steam. However, there has been only limited interest in shipboard application of the SAGT concept. The high efficiency of SAGT engines presents many potential benefits including a decrease in fuel consumption and extension of ship range and/or the time interval between underway fuel replenishment. Additionally, SAFT technology offers high power-turnup ratios for projected pulse-power weapon systems, and reduced stack weight, vertical moment, and IR signature.
An important concept used frequently in this disclosure is the Cheng point, described in U.S. Pat. No. 4,297,841, issued to Cheng and incorporated herein by reference, which corresponds to a SAGT engine operating at the point of maximum thermal efficiency or peak efficiency point (i.e., the so-called Cheng point). The Cheng point occurs approximately when the steam generated by exhaust gases in a waste-heat boiler becomes saturated.
Another SAGT system is disclosed in U.S. Pat. No. 4,509,324 issued to Urbach et al. on Apr. 9, 1985, and incorporated herein by reference. That system is based on a steam-augmented gas-turbine engine which includes a heat-recovery system (boiler), an intercooler, and a water-purification system.
FIG. 9 illustrates some basic concepts of SAGT systems wherein a first compressor 90 receives ambient air at its inlet, compresses the air and discharges the compressed air to an intercooler 91. The compressed air is cooled by the intercooler 91, which receives relatively pure water (i.e., having less than 0.20 parts-per-million solids) at a compatible pressure. The cooled compressed air is discharged from the intercooler 91 to a second compressor 92. The output of the second compressor is input to a combustor 93 and is used to burn fuel in the combustor in the presence of steam produced by a waste-heat boiler 94 and injected into the combustor by a steam injection means (not illustrated). The amount of fuel, steam, and compressed air introduced into the combustor is regulated in a predetermined manner to ensure operation of the engine system at a predetermined point (e.g., the stoichiometric point which is defined as the operating point where fuel and air are joined in a one-to-one chemical ratio so that all oxygen is consumed) and along a predetermined power profile curve. The use of steam in SAGT systems generally yields a specific power that is approximately threefold greater than the specific power of simple-cycle engines because a relatively negligible amount of energy is used to compress water in its liquid state. In the SAGT engine concept described above, steam/water flow rates up to 50% by weight of the air flow rate are utilized.
Thus, using up to 50% steam and/or water mixtures increases gas-turbine engine power by a factor of three or more, while maintaining the same air flow. Thus, at constant power, the total air demands are advantageously reduced and hence the size and number of gas turbine units for a given power requirement may be reduced accordingly.
The output of the combustor drives the compressor turbine 95, which drives the first and second compressors 90, 92, and a power turbine 96, which drives a load (not shown). The exhaust from the power turbine is cooled in the waste-heat boiler 94 which evacuates the exhaust to a stack and ultimately out to the ambient atmosphere. The waste thermal energy from the exhaust is used to generate steam to be input to the combustor. As discussed above, the source of energy for the injected steam and/or steam-water mixture is waste heat extracted from the effluent stack gas.
Since, as mentioned above, the energy required to compress the water is negligible, the available specific power of water is typically about 1000 hp-sec/lbm as opposed to less than 190 hp-sec/lbm for air in a simple-cycle engine. With maximum steam/water augmentation, a threefold increase in power output is achieved without any increment of air flow. At constant power, a threefold reduction in air requirements is realized. Consequently, the stack volume of intake and uptake ducts mentioned above can be substantially reduced in a SAGT engine.
Naval application of SAGT systems demands a large water-purification system, which had been considered a major technical impediment to development of a marinized version of the steam-augmented gas-turbine power plant. However, studies of water purification based on a reverse-osmosis plant have shown that less than 4200 cubic feet of space may be required to operate the SAGT engines for a destroyer with a 100,000-hp propulsion power plant. Thus, this is not a significant problem.
However, because of the wide range of mass flows necessary in the SAGT engine, difficulties arise in the previous disclosed engine from losses incurred by flow regimes which deviate severely from optimum design-point conditions. As a consequence, although the overall thermodynamic efficiency is always better than simple-cycle efficiency, the goal of high efficiency over the entire range of engine power is not always attained. This is a problem.
Further, the intercooler shown in FIG. 9 helps to increase specific power and efficiency by increasing the density of air entering the high-pressure compressor. Therefore, the annular flow area of the high-pressure compressor must be smaller to match the flow requirements. Since these changes significantly alter the characteristics of the new machine, it is very difficult to manufacture the engine from off-the-shelf components.
Further, while some commercial versions of steam-augmented gas turbines accept steam in amounts up to 16% of the compressor air flow, high levels of steam injection are avoided because commercial turbines, originally designed as simple-cycle machines with smaller flows, often undergo surge, hazardous overspeed, and serious off-design losses with greater steam injection.